Middle East Respiratory Syndrome (MERS) and Severe Acute Respiratory Syndrome (SARS) are coronavirus-induced human respiratory diseases with high case-fatality rates. Disease is especially severe in aged populations. In the previous funding period, we showed that age-dependent increases in prostaglandin D2 (PGD2) and an upstream phospholipase A2, PLA2G2D contributed to poor immune responses and decreased survival. The lung is in a state of chronic inflammation, resulting from continued exposure to environmental antigens. We postulated that PLA2G2D, which has anti- inflammatory properties, is upregulated to counter this low grade inflammation, resulting in delayed responses to innocuous antigens but also to rapidly replicating viruses like MERS-CoV and SARS- CoV. In contrast, genetic absence of DP1, the PGD2 receptor on myeloid cells, appears to result in poor respiratory dendritic cell activation suggesting that PGD2-DP1 signaling may have pro- inflammatory properties at early times after infection. Our central hypothesis is that small lipid mediators are major factors in the inflammatory milieu in the lung, affecting many aspects of the immune response to MERS-CoV, SARS-CoV and other respiratory pathogens. This hypothesis will be approached in the following specific aims: 1. To determine the mechanism of PLA2G2D upregulation and the role of PLA2G2D in vaccine responses in 12m old mice. CoV replication includes extensive cellular membrane rearrangements. The role between these rearrangements, the induction of oxidative stress and the upregulation of PLA2G2D will be investigated. 2. To determine the role of PGD2-DP1 signaling in the immune response to SARS-CoV in 12 m mice. The absence of PGD2-DP1 signaling results in diminished rDC activation and type I IFN (IFN-I) expression and increased inflammasome activation. Our goal is to determine whether changes in inflammasome activation are the major pathogenic effect of absent PGD2-DP1 signaling or if other factors are also involved. 3. To determine whether disease severity in murine MERS is age-dependent and whether PGD2 and PLA2G2D contribute to poorer outcomes. Using our newly developed hDPP4-KI mice and mouse-adapted MERS-CoV, we will determine whether MERS-CoV in mice also causes an age-dependent disease. We will also assess whether changes in eicosanoid expression contribute to more severe disease. MERS-CoV, unlike SARS-CoV, productively infects macrophages. In this aim we will determine whether productive infection of human and murine macrophages modulates PLA2G2D expression.